1) Field of the Invention
The present invention relates to a ceramic filter used for filtering off nonmetallic inclusions mixed in molten metal, particularly molten aluminum.
2) Description of the Prior Art
When aluminum (including aluminum alloy) is melted, various nonmetallic inclusions usually mixes in it. The inclusions are removed from molten aluminum by means of filters.
A ceramic filter used for this purpose is disclosed in Japanese Patent Provisional Publication gazette No. 22327/1977, which is made by sintering fused alumina for an aggregate and an inorganic binder containing 10 to 50 wt % of SiO.sub.2 and 5 to 20 wt % of B.sub.2 O.sub.3.
There is a problem with this filter that its constituents such as Ca and Si dissolve into molten aluminum while molten aluminum is passed through it. Though the amount of dissolution is very small, this problem is serious particularly to high purity aluminum.
Another type of ceramic filter which does not contain SiO.sub.2 is proposed in Japanese Patent Provisional Publication gazette No. 34732/1990. The binder of this filter comprises 15 to 80 wt % of B.sub.2 O.sub.3, 2 to 60 wt % of Al.sub.2 O.sub.3, and 5 to 50 wt % of CaO or MgO or both of them. Though there is no problem of the dissolution of the constituents such as Si with this filter, it can break during use because of the lack of strength of the filter itself. Further, since this filter does not contain SiO.sub.2, and in general the filters which do not contain SiO.sub.2 have a poor wettability for molten aluminum, its permeability for molten aluminum is low, and the filtering capacity of this filter is not sufficient for practical use. The low permeability for molten aluminum also causes the rise of the initial head of molten aluminum (the height of the surface of molten aluminum when the molten aluminum is poured on the filter at the beginning of a filtering operation and begins to pass through the filter). Therefore, when conventional filters being used in an existing apparatus are replaced with this filter, the filtering apparatus sometimes must be modified so as to accommodate the rise of the initial head.